Field of the Invention.
The present invention relates to ferromagnetic thin film memories and, more particularly, to ferromagnetic thin film memories in which states of the memory cells based on magnetization direction are determined through magnetoresistive properties of the thin film sensed by an electronic circuit
Digital memories of various kinds are used extensively in computers and computer system components, in digital processing systems, and the like. Such memories can be formed, to considerable advantage, based on the storage of digital bits as alternative states of magnetization in magnetic materials in each memory cell, typically thin film materials. These films may be ferromagnetic thin films having information stored therein through the direction of the magnetization occurring in that film, this information being obtained through either inductive sensing to determine the magnetization state, or by magnetoresistive sensing of such states Such ferromagnetic thin film memories may be conveniently provided on the surface of a monolithic integrated circuit to provide easy electrical interconnection between the memory cells and the memory operating circuitry.
Ferromagnetic thin film memory cells can be made very small and packed very, closely together to achieve a significant density of stored digital bits, properties which permit them to be provided on the surface of a monolithic integrated circuit as indicated above. Suitable constructions are indicated in the above indicated references. One of those constructions, as an example, is shown in FIG. 1 where a bit structure, 10, for a memory cell is presented formed over a semiconductor material body, 12, as used in a monolithic integrated circuit, and directly on an insulating layer, 13, itself supported on a major surface of body 12 in the integrated circuit. Only a small portion of the integrated circuit is shown, and then only a small portion of the semiconductor body is shown in that integrated circuit portion.
These bit structures in an assemblage in a memory are usually provided in a series string of such bit structures, there being a plurality of such strings. In order to make interconnections between members of such a string, or between the string and the external circuitry in the integrated circuit for operating the memory, terminal regions or junctures, 14, are provided at each end of the bit structure for interconnection purposes. These interconnections might be formed of copper alloyed in aluminum.
The remainder of the bit structure disposed on the exposed major surface of insulating layer 13 is comprised of a lower ferromagnetic thin film, 15, and an upper ferromagnetic thin film, 16. Ferromagnetic thin film layers 15 and 16 exhibit uniaxial anisotropy, magnetoresistance, little magnetostriction, and are of an alloy composition typically comprising nickel, cobalt and iron.
Between ferromagnetic thin film layers 15 and 16 is a further thin layer, 17, which usually would not exhibit ferromagnetism but may be either an electrical conductor or an electrical insulator. Layer 17 must, however, in this construction, prevent the exchange interaction between electron spins on neighboring atoms from coupling across between layers 15 and 16 to lock together the magnetization vectors of each. A typical choice for layer 17 would be silicon nitride. An insulating layer, 18, covers bit structure 10 although only a part of it is shown in FIG. 1. This "sandwich" structure is used to reduce magnetic fields outside of bit structure 10.
Finally, a word line, 19, is shown in FIG. 1 disposed on the major surface of insulating layer 18. Word line 19 typically comprises an aluminum layer alloyed with copper on a titanium-tungsten base layer. A protective and insulating layer over the entire structure of FIG. 1 would be used in practice but is not shown here.
Bit structure 10 can be operated in a longitudinal mode having its easy axis extend between internal interconnections 14 perpendicular to the direction of word line 19, or in a transverse mode having its easy axis of magnetization parallel with the direction of word line 19. In either situation, information, kept as a digital bit having one of two alternative logic values in bit structure 10, is stored therein in layers 15 and 16 by having the magnetization vector point in one direction or the other, generally along the easy axis of magnetization. If the direction of magnetization is caused to rotate from such a direction by external magnetic fields, the electrical resistance of layers 15 and 16 changes with this magnetization direction rotation because of the magnetoresistive properties of such layers. For the kinds of materials typically used in layers 15 and 16, the maximum change in resistance is on the order of a few percent at most of the minimum resistance value.
Sense current refers to the current flow through bit structure 10 from one terminal 14 to the other terminal 14 thereof, and word current refers to current flowing in word line 19 adjacent to, and transverse to the orientation of, bit structure 10. Bit structure 10 can be placed in one of the two possible magnetization states through the selective application of sense and word currents, i.e. information can be "written" in bit structure 10. A bit structure 10 of a typical configuration can be placed in a "0" magnetization state by the application of a sense current in one direction therethrough, of typically 2.0 mA to 3.5 mA, and coincidentally with provision of a word current from 10.0 mA to 30.0 mA. The opposite magnetization state representing a "1" logic value can be provided through providing the same word current and a sense current of the same magnitude in the opposite direction through bit structure 10. Such states will occur very quickly after the proper current levels are reached, such state changes occurring in less than about 10 ns.
Determining which magnetization state occurs in bit structure 10, i.e. reading the information stored in bit structure 10, is done by providing externally caused magnetic fields in that bit structure, through providing coincident sense and word currents, to rotate the magnetization of the structure. As indicated above, differences occur in the electrical resistance encountered between terminal regions 14 in bit structure 10 for different magnetization directions in the structure, including changing from one easy axis direction magnetization state to the opposite direction state. As a result, there will be differences in the voltage developed across magnetic bit structure 10 by the sense current flowing therethrough depending on the magnetization direction in this structure, and so depending on which magnetization state is present in bit structure 10.
Thus, to read information in a bit structure 10, a sense current through this structure is provided in the same direction as it was provided when a "0" magnetization state was set therein. A word current in the direction used in writing either a "0" or a "1" is again applied in that direction. This word current can be of a significantly smaller magnitude than used during writing if the information in the bit structure is not to be destroyed or lost during readout. However, this will lead to operating with a smaller resistance difference between "0" and "1" magnetization states and so a smaller output signal. A word current in the writing range of from 10.0 mA to 30.0 mA will lead to either a continued "0" state or an initial "1" state becoming thereafter a "0" state, but will provide the maximum resistance difference between states and so the largest output signal.
Thus, referring to the example of FIG. 2A, a bit structure 10 in the "0" magnetization state exhibits the resistance versus applied word current characteristic shown there for a sense current present of from 2.0 mA to 3.5 mA. The characteristic can be seen to have a generally positive slope. FIG. 2B shows the resistance characteristic of the same bit structure 10, in the same circumstances, but which has, instead, a "1" magnetization state written therein. In this latter situation, bit structure 10 exhibits a higher resistance versus word current at lower word currents, up to a breakpoint, which is indicated by the more positively sloped resistance characteristic. At the breakpoint, the "1" magnetization state is written to a "0" magnetization state just as in the writing of states described above. This causes the resistance characteristic to revert to essentially that shown in FIG. 2A.
Thus, if the word current is allowed to be sufficient to change a "1" state to a "0" state, the readout process will be a destructive one leading to the loss of information which would have to be replaced in a subsequent refresh writing operation. On the other hand, as can be seen from FIG. 2B, the maximum resistance change occurs at the breakpoint giving the peak voltage change across bit structure 10 at that point. This is the maximum signal voltage available for indicating the presence of a "1" state. The application of word currents less than that at the breakpoint in FIG. 2B will still provide a voltage change across bit structure 10, but of a magnitude less than the peak signal available. Such a nondestructive readout eliminates the need for a later refresh operation.
The arrows shown along the resistance characteristics in FIGS. 2A and 2B are those showing the resistance change locus which will occur in providing a word current pulse going from 10.0 mA to 30.0 mA and back to 10.0 mA. FIG. 2B also shows a dashed line arrow indicating the abbreviated resistance change locus which is followed for word line current less than those occurring at the breakpoint in FIG. 2B. Note that the change in resistance values are measured in tenths of ohms out of a hundred ohms or so. A 0.3 .OMEGA. change with a sense current of 3.5 mA means that the voltage change peak is only on the order of 1.0 mV over a nominal bit structure voltage of 0.3 Volts.
Thus, an electronic sensing circuit is desired which can sense this small signal. This sensing must occur in the face of differences occurring in bit structures 10 from structure to structure since such structures cannot be made absolutely identical although they can be made very similar to one another.